1. Field of the Invention
The invention relates to a safety device in a radiology machine and, more particularly, to a device for regulating the pressure of the cooling fluid of the X-ray tube in an X-ray unit.
An X-ray tube is constituted by a cathode and an anode enclosed in a vacuum-sealed jacket that enables electrical insulation to be set up between these two electrodes. The cathode is supplied with high voltage to produce an electron beam impinging onto the anode on a small surface which constitutes the focal spot of emission of the X-rays.
During its operation, the X-ray tube produces a large amount of heat, for only a small proportion of the energy used to produce the electron beam between the cathode and the anode is converted into X-rays, the rest being converted into heat. In an X-ray unit, to dissipate this heat, the X-ray tube is enclosed in a protection chamber or casing. Between the tube and the internal wall of the casing, there flows a cooling fluid which gets heated upon contact with the tube, before passing again into a circuit where it is itself cooled in a heat-exchanger, of the air or water type for example.
This fluid contained in the chamber is subjected to high rises in temperature, leading to an expansion of its volume, hence to a possibility of excess pressure inside the chamber when the tube works outside its normal range of operation. However, the pressure of the fluid cannot go beyond a limit threshold of about 4 bars, or else there will be deterioration of the X-ray unit.
2. Description of the Prior Art
There are two types of solutions envisaged, at present, to solve the problems of the accidental occurrence of excess pressure. These are, firstly, to permit a greater volume of expansion of the cooling fluid and, secondly, to monitor the pressure and the temperature of this fluid.
According to the first type of solution, the chamber that protects the X-ray tube is provided with an elastic membrane, permitting variations in the volume of expansion of the cooling fluid during the normal operation of the tube. However, during great increases in temperature, which give rise to increases in the volume of expansion of the fluid exceeding the limit permitted by the membrane, there arise either risks of the tearing of this membrane resulting in the release of the entire volume (namely about ten liters) of the hot fluid in the vicinity of the patient and the radiologist, or risks of the bursting of the tube, which would damage the X-ray unit, which may thus become dangerous.
Furthermore, the present trend is to reduce the dimensions of the X-ray unit to the maximum, preventing an increase in the volume of expansion. As for the choice of a rigid and closed chamber to place maximum limits on the splashing of liquid, this proves to be more dangerous in the event of a high degree of excess pressure of the fluid in the chamber.
In order to prevent such risks, the second type of solution provides for safety devices comprising pressure sensors or temperature sensors for the cooling fluid. However, owing to the continual development of X-ray tubes towards ever greater power values and, above all, towards ever greater thermal capacities for the anode, associated with a limitation of the space occupied by the X-ray unit, the risk of excess pressure of the fluid in the casing has considerably increased. Thus, in the case of a relatively lengthy X-ray examination during which the temperature of the cooling fluid is close to its upper limit value and the heat stored in the anode is at its maximum, any stopping of the cooling process, due to a current failure for example, will prompt a major increase in the temperature of the fluid which could be detrimental, even if the above-mentioned safety devices have worked perfectly. Indeed, since the anode is at its maximum temperature, it will release the heat stored by radiation towards the fluid which is no longer cooled. If, in addition, the X-ray tube breaks at this precise instant, the heat of the anode is instantly yielded to the fluid. It is observed then that all that the safety systems have done is to cut off the power supply of the X-ray tube: they have not prevented the risks of excess pressure of the fluid in the protective chamber.